The invention relates to a process for electroslag remelting of metals, in particular of iron-based and nickel-based alloys, for producing remelting blocks from one or more consumable electrodes in a short, water-cooled sliding ingot mould according to the preamble of patent claim 1. In addition, the invention includes an ingot mould which is improved with respect to the state of the art for carrying out this process.
In electroslag remelting plants operating today, water-cooled ingot moulds are used for shaping and producing the remelting blocks, their casting mould wall shaping the block and holding the slag bath generally consisting of copper, since this material, as also known from continuous casting, is most suitable to remove the quantities of heat being released on solidification of metals rapidly and efficiently into the cooling water. Whereas in continuous casting, only short ingot moulds are used, from which the solidifying bar is withdrawn more or less continuously after forming a first supportable bar shell, in electroslag remelting according to the state of the art, both the use of so-called fixed ingot moulds and the use of short sliding ingot moulds is conventional.
In fixed ingot moulds, the length of the ingot mould corresponds to the length of the block to be produced. The ingot mould is filled here successively with remelted metal in the course of the remelting process by melting out the self-consuming electrode in the slag bath floating on the metal surface, wherein there is no relative movement between ingot mould—or ingot mould wall—and remelting block.
When using sliding ingot moulds, remelting blocks are produced, the length of which exceeds the length of the ingot mould by a multiple. The short, water-cooled ingot mould serves here as a melt and casting mould, in which the hot slag bath is situated and in which the metal melting out from the electrode is collected and solidified subsequently to form the remelting block. The ingot mould is therefore required for carrying out the remelting process only in the region of the slag bath and in the region of block solidification. If the remelting block is solidified one time, the casting mould fulfils no further purpose. It is thus possible to restrict the length of the ingot mould to this range described above and to withdraw the block being formed during the melting out process, for example from the ingot mould at an average rate which corresponds to the rate of block construction. This leads to a relative movement between the block formed and the ingot mould wall and results in the meniscus of the metal surface and the slag bath resting thereon—except in the start-up phase—remaining essentially at a constant level within the ingot mould during the entire block construction. Instead, as described above, of withdrawing the block being formed from an ingot mould installed in a working platform, it is also possible to construct the remelting block on a fixed base plate and to withdraw upwards the short ingot mould by means of a suitable device at a rate corresponding to the rate of block construction.
As generally known, the consumption of electrical melting energy during electroslag remelting is relatively high compared to other melting processes likewise operating using electrical energy, such as for example during scrap-metal melting in an electric-arc furnace or a crucible induction furnace, since during electroslag remelting, the melting out rate is controlled primarily in order to ensure fault-free solidification structure of the remelting blocks. An energy saving by increasing the melting-out rate is therefore not possible, wherein direct contact of the slag bath heated to high temperature by the passage of current and serving as a heat source with the water-cooled ingot mould wall still has a considerable additional negative influence.
In order to melt iron-based or nickel-based alloys from ambient temperature and to heat them at about 1,600° C., a theoretical energy requirement of about 400 kWh/t is necessary. If the melting and superheating takes place in an electric-arc furnace or induction furnace using only electrical energy, an energy consumption of 500 to 700 kWh/t can be expected due to the process-related heat losses. In contrast thereto, the energy consumption in the production of a remelting block having, for example 1,000 mm diameter for a remelting rate of 1,000 kg/h depending on the slag used and the level of the slag bath, is between 1,000 and 1,800 kWh/t. This can be attributed to the fact that the heat flow from the hot slag via the water-cooled ingot mould wall into the cooling water, depending on slag composition, is between 1,000 and 2,000 kW/m2. For the most frequently used slags having in each case ⅓ CaO, CaF2 and Al2O3, about 1,100 kW/m2 must be expected. For remelting in an ingot mould having 1,000 mm diameter and a slag bath level of about 200 mm, an energy loss in the slag zone of about 630 kW must thus be expected, which for a remelting rate of 1,000 kg/h corresponds to an energy consumption of about 630 kWh/t. Relative to a total energy consumption during remelting with the slag indicated above of about 1,300 kWh/t, this corresponds to a percentage of just 50%. For slags having higher contents of CaF2, this proportion may however become significantly higher still.
For the reasons outlined above, it would therefore be obvious to insulate the ingot mould against heat losses in the region of the slag bath in order to thus lower the melt energy consumption. According to the Austrian patent specification 287 215, it has already given a corresponding proposal in 1968, according to which during electroslag remelting, by controlling the position of the surface of the metal melt in the ingot mould, the entire slag floating on the metal melt is collected as a liquid slag layer in a heat-insulated zone of the ingot mould; the temperature of the liquid slag layer is thus kept above or at least at the melt temperature of the metal by the heat insulation. The liquid metal collected in the ingot mould therefore passes into the region of the heat insulation, and the line of separation between the insulated and the water-cooled ingot mould part is situated below the metal surface.
This arrangement corresponding to the current state of the art has the disadvantage that the start of solidification is not adequately defined and hence considerable difficulties may occur in operational use. Hence, it is possible that with corresponding superheating of the metal, the latter penetrates into the gap between insulated and water-cooled part of the ingot mould and solidifies there in contact with the water-cooled lower part and forms a metal tab which remains suspended in the gap. Depending on the thickness of the tab, the block may now remain suspended anyway in the ingot mould and block withdrawal may become impossible, as a result of which the remelting process would be stopped. If the tab is of lower thickness and is not formed over the entire block periphery, cracks will be formed in the solidifying shell, which make difficult at least further processing of the block. If deeper cracks are formed, there may be discharging of liquid metal and the slag, as a result of which the remelting process would be stopped again. These problems of transition from an insulated container to a water-cooled form of solidification in contact with liquid or in the end solidifying metal are known from horizontal continuous casting. There, the problem is solved in that a so-called breaking ring made from boron nitride, which prevents advance of solidification beyond the boundary line insulation-water-cooling and facilitates easy release of the solidified bar shell due to its specific properties with regard to heat conductivity, wettability by the liquid metal etc., is installed at the transition point insulation-water-cooling. However, boron nitride is an expensive material which is complex to produce and can be obtained only in relatively small dimensions up to conventional continuous casting dimensions in the range up to about 200 mm diameter and is thus not suitable for the dimensions of 500 mm diameter and considerably above that which are of interest for electroslag remelting. For all these reasons, the process outlined above corresponding to the state of the art has as yet not found a practical application in spite of the obvious economic advantages.
The object of the present invention is now to utilize on the one hand the economic advantage of thermal insulation in the region of the slag bath during electroslag remelting, while the problems described above, so that a technical application becomes possible in useful manner.